The present invention is related to the field of data communications, and more particularly to the transmission of local area network data traffic through a synchronous communications network.
Traditional data communications among computers has been carried out using local-area networks (LANs). Ethernet LANs, for example, have been very widely used in the data communications field. Generally, data communication over LANs has employed data units or “packets” having a variable size. Explicit address information is included in a header portion of each packet to identify the recipient. Specialized control logic at the various nodes of a network are responsible for detecting incoming packets, temporarily storing them in variable-size buffers, and determining from the address information which node or nodes the packet is destined for.
As LAN technology has developed, there has also been development in the technologies used in traditional telephony communications. Pulse code modulation (PCM) of voice signals and synchronous time-division multiplexed (TDM) transmission channels have been in use for many years. With advances in glass fiber technology, communications carriers have devised very high data rate signals for fiber optic transmission that carry thousands of individual TDM channels using hierarchical TDM techniques. In particular, a set of standardized synchronous transport signals (STSs) are utilized in networks that adhere to Synchronous Optical Network (SONET) standards. Synchronous digital signals having data rates ranging from about 51 Mb/s to over 100 Gb/s are defined in SONET, each signal generally incorporating an integer number of basic “STS-1” signals.
There has been increasing interest in and need for communications equipment that can interface with traditional and emerging LANs, on the one hand, and the high-speed synchronous communications networks of the type traditionally deployed in telephony communications. The telephony networks, for example, are used for inter-LAN communications in wide-area networks (WANs), and therefore special interfaces are required to translate between LAN hardware and protocols and the hardware and protocols of the synchronous networks. Additionally, SONET-compliant equipment has been incorporated into portions of private and semi-private networks where the expense of such equipment is justified by the performance it provides, for example in backbone segments that are required to carry very high volumes of data traffic.
In these hybrid networks, there has been increased use of techniques that can be classified as “LAN emulation”. In a typical application, two or more disjoint LAN segments, for example segments residing in different buildings, are connected by one or more high-speed network segments of the type traditionally used in longer-haul networks such as the telephony networks. For example, one or more fiber optic links carrying SONET traffic may be used for such a high-speed link. The equipment that interfaces the separate LAN segments to the high-speed segments operates such that the collection of separate LAN segments appear to the connected host computers as a single LAN. This type of operation has numerous benefits, including the protection of investments in LAN hardware and software while providing greater connectivity and network capacity than would be possible using LAN technology alone.
In some networks of the type described above, the high-speed segments may provide data transport services to a number of different sets of users. For example, different businesses within a building or complex may utilize the high-speed network. There may be several independent emulated LANs, for example, that share the use of the high-speed network. In such cases, it may be necessary to allocate the usable capacity or bandwidth of the high-speed network among these multiple entities, to ensure that each enjoys a specified capacity without regard to the use of the network by the other entities.
It has been known to provide data transport services for emulated LANs in Asynchronous Transfer Mode (ATM) networks. One advantage of ATM networks is the existence of a large virtual connection space. An 8-bit virtual path identifier (VPI) and a 16-bit virtual connection identifier (VCI) are used in each ATM cell to identify a particular virtual connection for the cell. Thus, a large number of virtual connections can be created and flexibly assigned to perform different functions in the network. ATM-based emulated LANs have exploited this capability by generously allocating different virtual connections for various purposes in emulated LANs. For example, separate virtual connections have been used for traffic between each distinct pair of devices. The virtual connection identifiers in such emulated LANs indirectly identify the sources and/or destinations of LAN data, simplifying the processing of LAN traffic at the boundaries between the ATM network and the LAN segments.
Synchronous, TDM networks such as SONET networks generally do not employ explicit connection identifiers such as those found in ATM networks. Rather, such networks typically employ hierarchical multiplexing schemes in which data is associated with a connection by virtue of the data's temporal position, or “time slot”, in a stream. Further, there are generally far fewer time slots in a given TDM stream than the maximum number of virtual connections in an ATM network. For example, the basic STS-1 signal of SONET can carry at most 28 distinct DS1 channels. The generous connection-allocation schemes used in prior emulated-LAN networks cannot be advantageously employed with such a coarse channel structure. A more efficient approach is required.